Selected or multiple reaction monitoring (SRM/MRM) are methods of absolute quantitation (also termed AQUA) in targeted proteomics, whereby a complex sample is spiked with a stable isotope-labeled peptide that acts as internal standard. Heavy peptides for each target must be synthesized and evaluated for suitability. Low-cost crude peptides are generally adequate for targeted assay development. Peptide libraries and quality standards thus identified can then be synthesized at AQUA purity for SRM assays.